1. Field of the Invention
The present invention relates to fluorescent lamps and to electronic ballasts of the type utilizing a DC output flyback converter.
2. Background Art
The use of fluorescent lamps has become widespread. The typical fluorescent lamp is composed of a glass tube containing an inert gas and a small amount of mercury. Phosphors coat the inside of the glass tube, and each end of the glass tube includes an electrode. In operation, a ballast provides current to the electrodes. A traditional ballast is a special transformer that uses electromagnetic principles to generate operating and starting voltages for fluorescent lamps. An electronic ballast uses electronics to achieve the same result. In operation, electrons migrate across the length of the tube, and excite the mercury atoms which are in a gaseous state. The arc releases photons in the ultraviolet band. The photons excite the phosphors that coat the inside of the glass tube, and the phosphors emit visible light. Fluorescent lamps are very efficient during operation. Before a fluorescent lamp can operate as described above, the lamp must be started, that is, the length of tube must be made conductive. There are several existing techniques for starting a fluorescent lamp.
One technique for starting a fluorescent lamp involves the use of electrodes that include filaments. Each electrode is composed of two conductive pins that connect to a filament wire including tungsten and boron. Preheating the filament at each end of the fluorescent lamp tube boils electrons from the filament to ionize the gas inside the tube. The ionized gas inside the glass tube is conductive, and needs a voltage across the electrodes to establish an electrical arc. Using preheating techniques for the filaments increases lamp life, enhances dimming performance and enhances cold operation performance.
Another technique for starting a fluorescent lamp is known as instant start. In instant start fluorescent lamps, a very high initial voltage is applied across the electrodes which are typically single pin electrodes. The high voltage causes a corona discharge where the gas inside the glass tube is quickly ionized and an electrical arc is established.
Background information relating to ballasts may be found in U.S. Pat. Nos. 4,958,108; 4,870,327; 5,969,484; 5,416,387; 4,184,197; 5,449,981; 5,428,268; and 5,814,976. In the operation of fluorescent lamps, two important operating parameters are crest factor and power factor. There is always a desire to make an electronic ballast that is more cost effective, and has improved operating parameters.
For the foregoing reasons, there is a need for an improved electronic ballast of the type having a DC output flyback converter.